humidity control by adjusting heat and reheat coils
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  1. #1

    Confused humidity control by adjusting heat and reheat coils

    Hello all,
    I am battling very dry air in my lab spaces, humidifiers working full blast and cannot seem to get the humidity levels up, I have an air handler equipped with a heat coil, cooling coil then a reheat coil, what should the setting or ration to one another be so I can control humidity, basically make sure I am not just condensing it all out of the air, what part does this system play in the humidity control and how can I optimize it? Any help would be greatly appreciated, thank you

  2. #2
    Join Date
    Oct 2009
    Location
    WI
    Posts
    114
    What is the fan setting?

    Reheat should only be used for dehumidification.

    You may want to look into a larger scale duct mounted humidifier. Wet mopping the floor in the evening also helps up the humidity.

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,232
    If cooling is needed, avoid condensing moisture on the cooling coil. If the outside air has a low dew point, reduce the amount of fresh air as much as possible.
    Where are located?
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  4. #4
    This is a makeup air handler, i need the outside air to keep positive pressure in the labs, and its 30f degrees outside so i need the reheat to warm the air, i thought most dehumidification was due to the preheat warming the air causing it to condense on the cold coils? I am looking for the best settings to avoid removing any mosture, if possible,
    thanks for your input

  5. #5
    Join Date
    Dec 2003
    Posts
    2,349
    You need more humidification. If its 30 deg outside the air has a dewpoint no more than 30 deg. So, you cant be removing humidity with your cooling coil unless you have it colder than 30 deg, which is unlikely.

    How do you know the humidifier is doing all it can do? You probably need to get someone to check it out. It is probably not functioning correctly.

  6. #6
    Join Date
    Dec 2009
    Posts
    161
    Quote Originally Posted by bigtime View Post
    You need more humidification. If its 30 deg outside the air has a dewpoint no more than 30 deg. So, you cant be removing humidity with your cooling coil unless you have it colder than 30 deg, which is unlikely.

    How do you know the humidifier is doing all it can do? You probably need to get someone to check it out. It is probably not functioning correctly.
    That is what I was thinking as well. Make sure they are operating properly. It is possible they were added after the building was designed and weren't sized properly to begin with. Maybe they need to look into Lieberts if it is a lab that needs a specific humidity setpoint?

  7. #7
    Join Date
    Apr 2010
    Location
    southern california
    Posts
    535
    The make up air unit should have some sophisticated controls. What brand of equipment are you using. Lab hvac systems can be rather complex. Any time outside air is introduced( usually 100%) humidity and temperature becomes harder to control because the outdoor conditions are constantly changing. A steam humidifier is required to maintain humidity control. Is it safe to guess that you are exhausting 100% of the makeup air. Enthalpy wheels would be beneficial. Reheat coils are for dehumidification, as the air is cooled leaving the evaporator it is reheated. This allows the air to absorb more humidity. All labs should maintain RH above 30% do to static discharge which is capable of starting a fire or explosion. Most labs are never designed properly from the beginning. Design conditions are sometimes not known and guessing will always end up with bad results. Determine your indoor design conditions and then one can solve the deficiencies. It is all about psychrometrics, in your case more grains of moisture are required. If you can reduce your outside air requirements, then your existing humidifier may work. A lot more info is needed to solve your problem correctly.

  8. #8
    Join Date
    Mar 2003
    Posts
    7,321
    before you go guessing at all of the answers posted so far, stop and think for a minute. The first question you need to ask is...did this ever work? if yes, then there are control issues, if no, well....., you may have biger issues than you think. just changing setpoint on a heating coil most likely isnt your answer.
    is this a true makeup air handler, 100% oustside air, or mixing box setup?
    what is the location of the dispersion tube?
    has anyone checked to make sure you have enough capacity?
    is it really working at 100%?

    I have seen many misapplied humidifiers in my day.

  9. #9
    Join Date
    Nov 2010
    Posts
    16
    What type of humidification is the unit designed for?

  10. #10
    Join Date
    Dec 2010
    Location
    Central US
    Posts
    412
    Definately need more info.

    What is your pre heat coil setpoint and actual temp?
    What is your space temp setpoint and actual temp?
    what is your space humidity setpoint and actual humidity?
    Do you have stick controls or automation controls monitored on a PC?
    What kind of control valves do you have, 2-way, 3-way, 2-position, modulating, manual or none. wing dampers or face and bypass dampers or what?

    Typically pre heat has modulating control valve with a pre heat sensor upstream of pre heat coil maintaining a setpoint between 60-65 deg f. Never to be changed or varied.
    Chilled water coil has modulating control valve that only modulates to maintain cooling setpoint or driven to a % open in dehumidification mode only. Otherwise it remains closed.
    Re heat coil typically has a modulating control valve that only modulates to maintain heating setpoint or % open in dehumidification mode until humidity level and or heating setpoint is satisfied.
    Humidifier typically has a modulating valve to maintain humidity setpoint in humidification mode only.

    Lots and lots of variables can effect the outcome of humidity, additional cooling or heating sources, the amount of exhaust air versus make up air, doors or windows being open etc..

    I would highly recomend an experienced professional to evaluate your issues. If you change any original design aspects it could make problems worse.

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